The present invention relates to an electric charging member and an electric charging apparatus suitable for charging a body to be charged such as a photosensitive body in an electrostatic latent image forming process performed in an electrophotographic system such as a copying machine or a printer.
In an electrostatic latent image forming process performed in an electrophotographic system such as a copying machine or a printer, an electrostatic latent image is formed by uniformly charging a surface of a photosensitive body, and projecting an image from an optical system on the photosensitive body to erase the electrification on a portion on which light is incident, to thereby form an electrostatic latent image. The electrostatic latent image is then stuck with toner to form a toner image, and the toner image is transferred, that is, printed on a recording medium such as paper.
In this case, th first operation of charging a photosensitive body has been generally performed using a corona discharge method. The corona discharge method is undesirable in terms of safety and maintenance of the machine because it requires application of a high voltage of 6 to 10 kV. The corona discharge method also presents an environmental problem because of occurrence of toxic substances such as ozone and NO.sub.x during corona discharge.
For this reason, attempts have been made to develop a new charging method capable of charging a body to be charged at a voltage lower than that required for corona discharge and of suppressing occurrence of toxic substances such as ozone. As such a new charging method, there has been proposed a contact-type method of charging a body to be charged by bringing an electric charging member applied with a voltage with the body to be charged at a specific pressure.
As the electric charging member used for the above contact-type charging method, there has been known a member formed by coating the surface of an elastic layer made from a rubber or urethane foam with a resin solution in which a resin such as a polyurethane or nylon is dissolved in an organic solvent by a dipping process or a spraying process, to form a covering layer of the resin such as a polyurethane or nylon for ensuring surface smoothness and preventing adhesion of toner.
In the contact-type charging method, however, since the electric charging member is in press-contact with a body to be charged such as a photosensitive body for a long period, the electric charging member is liable to be deformed, and if the deformed state is returned to the original one, there occurs an improper image. As a result, an elastic body and a coating constituting the electric charging member are required to be small in compression set.
Incidentally, in the contact-type charging method, a dc voltage is often superimposed with an ac voltage for stably keeping uniform electrifying charges on a body to be charged. In this case, however, when an ac voltage is applied in a state in which the electric charging member is in contact with the body to be charged, there occurs such a phenomenon that the body to be charged is tapped by the electric charging member by application of the ac voltage, leading to occurrence of noise. To cope with such an inconvenience, the hardness of the surface of the electric charging member is required to be reduced.
In this case, to reduce the compression set of the electric charging member, the elasticity of the coating may be made larger by crosslinking the resin for forming the coating; however, when the resin for forming the coating is crosslinked, the hardness of the coating becomes higher. As a result, there occur inconveniences that noise becomes larger and the surface of a body to be charged such as a photosensitive body in direct-contact with the coating is scratched.
On the other hand, to reduce the hardness of the electric charging member, a plasticizer or a softening agent may be added to a rubber material or a foam material for forming the elastic layer. In this case, however, there occurs a problem that the added plasticizer or softening agent will possibly bleed, leading to contamination of the body to be charged such as a photosensitive body.
In this way, according to the conventional methods, it is difficult to reduce noise while keeping an excellent charging performance by making prevention of deformation compatible with reduction in hardness for the electric charging member.